"Owlelephant sin(theta)" tells you that "owl cross elephant" means the cross product of two 3-D vectors. Cross product requires that both operands are vectors: you cannot calculate a cross product if an operand is scalar (not a vector), which sounds like scaler (someone who scales mountains). The punchline recalls learning about cross products in maths class, where the instructor kept saying "you can't calculate A x b because b is scalar".

I particularly like the joke because it combines two genres: math jokes and elephant jokes. However, it does have a limited audience and I had to have it explained to me the first time too.

Well you've lost me. I even tried saying them out loud in an American accent (I'm British). Still nothing. Them seem to rely on some word that sounds a bit like owl. Obviously, it won't be funny if you have to explain it, but I'm curious nevertheless.